Conventionally, as radio frequency power amplifiers for amplifying a modulation signal including an envelope fluctuation component, class A or class AB linear amplifiers have been used in order to linearly amplify the envelope fluctuation component. Such class A and class AB linear amplifiers provide a high linearity, but constantly consume power which accompanies a DC bias component and so have a lower power efficiency than, for example, class C, D or E nonlinear amplifiers. This brings about a drawback that when such a radio frequency power amplifier having a high power consumption is used in a mobile wireless apparatus having a battery as a power source, the battery life is short. When such a radio frequency power amplifier is used for a base station apparatus of a wireless system including a plurality of high power transmission apparatuses, the scale of the base station apparatus is enlarged and the amount of heat generation is increased.
In light of the circumstances, methods for improving the power efficiency using polar modulation have been conventionally proposed. FIG. 11 is a block diagram showing a structure of a conventional transmission device using a polar modulation system. As shown in FIG. 11, the conventional transmission device includes an amplitude/phase separation section 61, an amplitude modulation signal amplifier 62, a frequency synthesizer 63, and a radio frequency power amplifier 64 as a nonlinear amplifier.
The amplitude/phase separation section 61 separates an input baseband modulation signal S10 into a baseband amplitude modulation signal S11 and a baseband phase modulation signal S12. The amplitude modulation signal amplifier 62 performs predetermined amplification on the baseband amplitude modulation signal S11, and then supplies the resultant signal to the radio frequency power amplifier 64 as a supply voltage. The frequency synthesizer 63 performs phase modulation on a carrier wave signal with the baseband phase modulation signal S12 to obtain a radio frequency phase modulated signal S13, and transmits the radio frequency phase modulated signal S13 to the radio frequency power amplifier 64. Thus, the radio frequency power amplifier 64 amplifies the radio frequency phase modulated signal S13 under the supply voltage in accordance with the baseband amplitude modulation signal S11, and outputs the resultant signal as a transmission output signal S14.
Now, an operation of the transmission device using the polar modulation system will be described. Where the baseband modulation signal S10 is Si (t), Si (t) is represented by expression (1). Here, a(t) represents amplitude data, and exp [jφ(t)] represents phase data.Si(t)=a(t)exp[jφ(t)]  (1)
The amplitude/phase separation section 61 extracts amplitude data a(t) and phase data exp[jφ(t)] from Si(t). The amplitude data a(t) corresponds to the baseband amplitude modulation signal S11, and the phase data [jφ(t)] corresponds to the baseband phase modulation signal S12. The amplitude data a(t) is amplified by the amplitude modulation signal amplifier 62 and is supplied to the radio frequency power amplifier 64. Thus, the value of the supply voltage of the radio frequency power amplifier 64 is set based on the amplitude data a(t).
The frequency synthesizer 63 generates the radio frequency phase modulated signal S13 by modulating carrier wave angular frequency ωc with the phase data exp[jφ(t)], and inputs the radio frequency phase modulated signal S13 to the radio frequency power amplifier 64. Where the radio frequency phase modulated signal S13 is signal Sc, signal Sc is represented by expression (2).Sc=exp[ωct+φ(t)]  (2)
Since the radio frequency power amplifier 64 is a nonlinear amplifier, the supply voltage value a(t) of the radio frequency power amplifier 64 is multiplied by the output signal from the frequency synthesizer 63, and the resultant signal is amplified by gain G to generate the transmission output signal S14. The transmission output signal S14 is output from the radio frequency power amplifier 64. Where the transmission output signal S14 is RF signal Srf, RF signal Srf is represented by the expression (3).Srf=Ga(t)Sc=Ga(t) exp[ωct+φ(t)]  (3)
The signal which is input to the radio frequency power amplifier 64 is a phase modulated signal which has no fluctuation component in the amplitude direction and so is a constant envelope signal. This allows a highly efficient nonlinear amplifier to be used as the radio frequency power amplifier 64, and therefore a highly efficient transmission device can be provided. The technologies using this type of polar modulation system are described in, for example, patent document 1 and patent document 2.
Patent document 1: Japanese Patent No. 3207153
Patent document 2: Japanese Laid-Open Patent Publication No. 2001-156554
Patent document 3: U.S. Pat. No. 6,191,653